terraformingfandomcom-20200214-history
Surviving A Supernova
Main article: Red Supergiants Approaching Supernova. A supernova is one of the most violent explosions in the Universe. It is triggered when a star runs out of energy. Can a planet survive a supernova? The answer could be yes or no, depending on many factors. In order to see if a planet can survive, we must understand what is happening in a star during a supernova event and what will a planet endure during the explosion. Explosion Waves The mechanism of a supernova is much more complex then it seems. Basically, a star sustain thermonuclear reactions, fusing what it has into heavier elements, until it reaches iron. At that point, fusion reactions no longer produce energy. The star cannot sustain its weight any longer. The core collapse. The energy from core collapse is mostly transformed into neutrino radiation. That is, 99% of the energy passes away from the star, unnoticed and without interacting with anything. Surrounding the core, matter (already fused up to the state of iron) falls in, to fill the empty space formed after core collapse. This matter, known as the iron wall, gets so compressed, that even neutrino radiation cannot easily pierce through. So, neutrinos have to lose a small amount of energy, pushing the iron wall away. This push actually triggers the explosion. The iron wall is not homogenous. Neutrinos will find holes in the wall and will escape most often through them. This will influence the explosion itself and make it asymmetrical. Just like an atomic bomb, which produces three destructive waves, a supernova produces multiple waves. # Neutrino wave # Gamma ray bursts (not always) # Light wave # Plasma wave. Neutrino waves: They last for roughly 10 seconds and are so powerful that can be recorded from Earth. They occur in the first 10 seconds after core collapse. Gamma ray bursts: They are formed when very massive stars explode and escape through the poles, shortly after neutrino waves. They last up to 20 minutes and are seen at 3 hours before visible supernova. They are very powerful and can destroy life on a planet located hundreds of light years away. Luckily, they are spread on a very narrow angle. Light waves: They are visible at 3 hours after core collapse and last until the end. They release a huge amount of energy, mostly as blue and ultraviolet light. Heat released is also significant. Plasma waves: As the outer layers of the star as pushed away, they are transformed into a very fast and very hot plasma wind, eroding all objects it encounters. Effect on the planet The fate of a planet is dictated by a few factors. The most important factor is distance to the star. *Supergiant Stars are very large. If we replace the Sun with Betelgeuse, which is a supergiant star, all planets up to Jupiter will be engulfed by it. A habitable zone around Betelgeuse is located within 180 AU. However, planets suitable for terraforming or paraterraforming could be located much closer or further away. *Wolf-Rayet Stars are also very large and bright. They don't have habitable zones, but might have planets suitable for colonization. *White Dwarfs can go supernova. Their habitable zones are very close, less then 1 AU, severely threatening the chances of a planet to survive. *Neutron Stars are thought to produce a supernova or at least a nova as they age. They don't have habitable zones, but are known to host planets. Each explosion wave will have a different and distinct effect on the nearby planets. Neutrino wave Neutrino radiation usually gets unnoticed. However, during a supernova, it becomes relevant. At 1 AU, it can be lethal. However, this applies only for white dwarfs. Neutrino radiation during a supernova can cause cancer or other diseases up to 7 AU from the explosion site. However, it can create spectacular shows of light up to 300 AU, as some neutrinos interact with hydrogen atoms. Water might appear glowing blue for a few seconds and it will heat. Because the iron wall is not homogenous, some planets might experience a much more powerful neutrino burst then others. At that intensity, neutrino radiation will seriously affect any electronic equipment. This can destroy ships that are trying to escape. Humans are highly sensitive to rises in temperature. On a planet located at 200 AU, the neutrino wave might rise human body's temperature from 36 to 46 degrees C, which is lethal. There is no known material able to deflect neutrino radiation. Gamma ray wave This occurs in case of massive stars. It is not expected to happen in case of Betelgeuse. As matter falls in, trying to fill the space where the core collapsed, it also starts to rotate. Spinning creates two depressions at the poles. There, part of the energy produced by compression can be ejected as two powerful gamma ray bursts, which pierce all the way to the surface. Planets usually orbit in equatorial plane. However, the gamma ray bursts can be partially reflected by matter like solar wind and can hit planets orbiting in equatorial plane. Supernova gamma ray bursts are very powerful and can threaten life at 1000 light years away. However, they are released only in two polar jets and last up to 20 minutes. Reflected by the surrounding solar wind, they have the power to sterilize life on all planets. However, life will still survive deep inside oceans or on the not illuminated hemispheres... for the moment. On a terraformed planet, if it is day time, all living organisms will be instantly fried. If it is night time, settlers will see incredibly beautiful auroras. The gamma ray wave occurs minutes after the neutrino wave. The light wave The values listed here are for the supernova expected when Betelgeuse explodes. The visible supernova starts at three hours after neutrino wave. This is the time needed for the shock wave to reach the surface. In case of white dwarfs and neutron stars, the light wave is produced instantly. Star's luminosity increases dramatically. The light turns from red (in case of red supergiants) into blue and ultraviolet, also carrying significant infrared. This wave is far more uniform then all the others, affecting all planets located at the same distance in the same way. The light wave sends a huge amount of energy, heating and vaporizing most planets. One calculation shows that if the Earth is made entirely of iron, which requires a huge amount of energy to vaporize, at its current orbit, it will be completely vaporized within 6 hours. However, the Earth is made of many other elements, which require less energy to be transformed into vapours. Suppose we put a planet made of iron with a similar diameter with Earth at the orbit of Neptune and replace the Sun with a star going supernova. That planet will resist 250 days, long enough to survive the light wave. Betelgeuse has its habitable zone located much further away. We can calculate that a planet in Betelgeuse's habitable zone will lose a layer of 4 km of soil if it is made of silicates. For gas giants or icy planets, the effect will be far greater, but still, they have a chance to survive if they are placed far away. Habitable zone planets orbiting white dwarfs will have no chance to survive. They will be vaporized within hours. The plasma wave The most devastating event of a supernova is the plasma wave. During this phase, a planet receives not only heat from light radiation, but also from solar wind, as the outer layers are pushed into space. It not only heats the planets, but also has a highly abrasive effect, pushing matter away from the surface. In case of habitable planets surrounding white dwarfs, the plasma wave arrives at less then an hour. In case of a habitable planet orbiting Betelgeuse, it might take two or three weeks. The plasma wave is not homogeneous. A planet can be lucky enough to fit within a hole in the plasma wave. In this case, the effect can be less powerful then during the light wave. Or, an unlucky planet can be hit by a dense jet of plasma, which can be over 10 times more destructive then the light wave. The plasma wave has an abrasive effect. It pushes away volatiles very fast. Given the temperatures, most surface rocks become liquid or gaseous. So, there is far less energy needed to remove surface rocks then it would be to heat and vaporize them. The plasma wave will have a very powerful effect on gas giants, literally blowing away their atmospheres. In case of icy planets, it will easily remove their exposed subsurface oceans. Rocky planets might have molten cores, which are also easier to remove then solid surfaces. It is impossible to calculate the erosion force of the plasma wind. Assuming a habitable planet orbiting Betelgeuse, the erosion will be usually 4 times higher then during the light wave. However, as shown above, the effect can be weaker or more powerful. We can estimate that a terraformed planet around Betelgeuse will lose a layer of 1 to 50 km of solid silicate rocks. Even if the planet has a liquid, molten core, it will not be completely disintegrated. Planets located further away might have an even better chance. After the supernova As shown above, planets orbiting close enough will not have a chance. Distant planets can survive a supernova. After the explosion, the star might be no more (for example after a pair-instability supernova). So, surviving planes might have nothing left to orbit. Even if there is a stellar remnant (black hole or neutron star), it will be far smaller then the initial star. Outer planets will be ejected on hyperbolic orbits. There is a small chance that an inner planet, massive enough, might still be orbiting the stellar remnant on a highly elliptical trajectory. In most cases, the stellar remnant does not have the same motion as the initial star. So, there is a chance that this motion will match the movement of a planet, ensuring that the planet will still orbit. Surviving planets will then slowly cool. Having a significant part of their surfaces eroded, they will look completely different from what they were before. We can imagine oceans of lava on their surface. In case of icy planets, we can see oceans of water starting to form an icy crust (if some water managed to survive) or the rocky core exposed. Can life survive? There is no way for life to survive on the surface of a planet witnessing a supernova. As shown above, a habitable planet orbiting Betelgeuse will lose at least 4 km of its surface, vaporized to space. An icy planet the size of Earth, with a subsurface ocean, might lose, in a worse case scenario, half of its radius. So, there is a chance that some water still can survive, forming an exposed ocean. Over time, an ice crust will form again, protecting its precious life, if it exists. Can humans survive? Can settlers survive a terraformed planet during a supernova? As shown above, a planet orbiting Betelgeuse in the habitable zone will survive. There are, however, a few risk factors. * The planet must have a thick enough crust, so that settlers can dig deep enough. * The planet must not be hit by a powerful neutrino burst (as human body temperature will increase too much and all electronics will be destroyed). * The planet must be lucky enough to fit in a hole of the plasma wind. * After the supernova, the settlers must not be beneath a lake of lava. After the supernova, settlers need to stay hidden for decades, if not centuries. The resulting nebula can produce deadly radiation, even after short exposures. If the stellar remnant is a neutron star, it can still produce deadly gamma ray bursts. Even black holes can generate dangerous radiations in their accretion disks. A rescue mission will be very difficult given the environment left after a supernova. Also, it will be very hard, if not impossible, to locate possible survivals hidden inside the crust, since no initial Geographic features will be left.